1. Field of the Invention
The invention relates to a communication system which has a function of detecting a fault and which can conduct a high-speed data transmission.
2. Description of the Related Art
FIG. 16 shows a block diagram of a network in which stations A, B, C and D are connected so as to form two loops of station A.fwdarw.station B.fwdarw.station C.fwdarw.station D.fwdarw.station A (loop F), and station A.fwdarw.station D.fwdarw.station C .fwdarw.station B.fwdarw.station A (loop R), and signals are bidirectionally transmitted.
This network has a duplicated configuration of loop F and loop R. Therefore, even when a fault such as disconnection of a line occurs in one of the loops, data transmission can continue using the other loop.
Each of stations A to D comprises a CPU, a transmitting unit and a receiving unit for loop F, and a transmitting unit and a receiving unit for loop R. For example, a signal transmitted from the transmitting unit of station A is received by the receiving unit of station B, and then transmitted from the transmitting unit of station B to the receiving unit of station C. In this way, a signal is sequentially transmitted in a forward direction to the stations in loop F. Transmission in the opposite (reverse) direction is accomplished using loop R.
In the network, station A is a host station, and the other stations (stations B to D) are satellite stations. The host station, i.e., station A, controls and monitors the network so as to cope with a fault of the network.
In a method which is conducted by station A to detect a fault of the satellite stations (stations B to D) and transmission lines 700 to 707, for example, the host station detects a fault on the basis of times occupied by satellite stations for transmission and reception processes, and intervals of transmission and reception processes. Thus, when a satellite station detects a reception of abnormal data, the satellite station informs the host station. In one method which may be conducted by a satellite station to detect a fault, received data is subjected to a parity check or a sum check.
In such methods, however, it is impossible to detect a fault, such as the disconnection of a transmission line, which occurs during the idle state shown in FIG. 17 (a period during which no transmission data exists on the transmission line is referred to as an idle time, and the transmission line is in the idle state during the idle time). In order to deal with this problem, the method described below is conventionally employed. In the method, signals to be transmitted to a transmission line are encoded with Manchester encoding or the like, or signals including the state of the transmission line and having a signal level that is inverted within a predetermined time (within a bit time) are transmitted, and a fault is detected when a signal which fails to change in level within the predetermined time (i.e., a signal having a coding scheme other than that specified) is received.
Next, the signal transmission between stations in accordance with a conventional technique using such an encoding system will be described in detail, using the transmission between station B 200 and station C 300 as an example.
FIG. 13 is a block diagram showing in detail a CPU 500 and a transmitting unit 600 for loop F of station B 200, and a receiving unit 800 and a CPU 900 for loop F of station C 300. In the figure, the transmitting unit 600 of station B 200 and the receiving unit 800 of station C 300 are connected to each other through a transmission line 701.
As shown in FIG. 16, for each of loops F and R, station B 200 comprises the receiving unit 6000, the CPU 500 and the transmitting unit 600. The numbers 200-204, 6000-6004, etc represent stations and units concerned with different embodiments, as explained hereinafter. In FIG. 13, among of these units, only the CPU 500 and the transmitting unit 600 for loop F are shown, and the receiving unit 6000 for loop F is not shown.
Similarly, for each of loops F and R, station C 300 comprises the receiving unit 800, the CPU 900 and the transmitting unit 8000. In FIG. 13, among of these units, only the receiving unit 800 and the CPU 900 for loop F are shown, and the transmitting unit 8000 for loop F is not shown.
In FIG. 13, 500 designates the CPU of station B 200, 600 designates the transmitting unit, 1000 designates an encoder for encoding data to be transmitted, and 103 designates a transmitter for converting information output from the CPU 500 into information which can be input to the encoder 1000. The transmitting unit 600 consists of the transmitter 103, and the encoder 1000.
The reference numeral 800 designates the receiving unit of station C 300, 900 designates the CPU, 1001 designates a decoder for restoring an encoded signal on the transmission line to the original form, and 109 designates a receiver for converting information output from the decoder 1001 into information which can be input to the CPU 900. The receiving unit 800 consists of the decoder 1001, and the receiver 109.
When a signal is to be transmitted from station B 200 to station C 300 through the transmission line 701, station B 200 transmits a signal encoded by the encoder 1000 for each bit, during both the idle time and the time of transmission of transmission data. In station C 300, a reception signal is decoded by the decoder 1001 to restore original information which has not yet been encoded even during the idle time.
In the case where a fault such as disconnection of the transmission line occurs in the transmission line, generally, a reception signal after the point of the fault generation is fixed to "0" or "1".
When the decoder 1001 detects a reception signal having a coding scheme other than that specified, for example, fixed to "0" (LOW level) or "1" (HIGH level), the decoder 1001 outputs a decoding error. The decoding error is input as an interruption signal to the CPU 900 of station C 300 so that the CPU 900 recognizes a fault of the transmission line.
Specifically, when encoding is conducted in accordance with Manchester encoding, data "1" changes "from H to L" and data "0" changes "from L to H" during a 1-bit data time as shown in FIGS. 14(a) and 14(b). Also when data "1" is consecutively transmitted as shown in FIGS. 15(a) and 15(b), a signal on the transmission line changes. In a normal state, a signal on the transmission line 701 also changes in the idle state. Accordingly, when a signal on the transmission line 701 is fixed to "0" or "1", the CPU 900 can recognize a fault of the network.
However, such encoding causes a signal change on the transmission line 701 to be rapidly conducted as compared with the case where a signal is not encoded. Consequently, the data transmission rate must be set to be a value lower than the threshold frequency f which is restricted by the physical properties of the transmission line 701 (in the case of Manchester encoding, f/2).
In a conventional communication system, when employing a method in which encoding is conducted for each 1-bit data, there arises a problem in that the data transmission rate must be set to be a value lower than the threshold frequency f which is restricted by the physical properties of the transmission line. When employing a method in which the interval of transmission and reception processes is monitored, the interval of transmission and reception processes must be set to a time shorter than a predetermined time. Since a fault of the transmission line such as disconnection of the line is detected on the basis of the value of the interval of transmission and reception processes, the employment of such a method produces problems in that the detection requires a prolonged time and that the load on the internal processing of the transmitting station is increased.
The message identifying a fault state in the transmitting station is transmitted from the transmitting station to the receiving station in the same manner as a transmission of ordinary data, whereby a problem arises in that the process of sending the message requires a prolonged time.
The invention solves these problems. Accordingly, it is an object of the invention to provide a communication system in which a detection of a fault of a transmission line, such as disconnection of the line, need not wait for the interval of transmission and reception processes and can immediately be conducted during the idle time. The data transmission rate is not lowered, and it is not necessary to periodically transmit transmission data, so that the load on the internal processing of the transmitting station need not be increased.
It is another object of the invention to provide a communication system in which a fault state of the transmitting station can immediately be reported to the receiving station.